Recently, use of natural energy such as solar power generation and wind power generation, development of new energy such as fuel cells, and energy saving of various things including vehicles and home appliances are actively progressed due to an environmental problem. Reduction of energy consumption and reduction of power consumption are important problems of Japan and the whole world in the future.
It is said that one of solutions thereof is measurement and display of power consumption of an electric apparatus and an electronic apparatus, and management control of storage and consumption of power. To realize this, a measurement apparatus having small size and light weight and a low cost is necessary.
For example, a large amount of harmonic current is generated in an inverter to convert direct-current power generated by an inverter power supply or a solar battery incorporated in an air conditioner, an electric vehicle (EV), and a hybrid electric vehicle (HEV) into alternating-current power of a commercial frequency.
In measurement of the harmonic current, the current is converted into a voltage by an ammeter (also referred to as “CT”: Current Transformer) and a shunt resistor, and the measurement using analysis (Fourier analysis) by a computer or a filter function by an electronic circuit is then performed.
In measurement of a high frequency/harmonic current, measurement precision greatly depends on a frequency characteristic (magnetic characteristic) of the CT used for the measurement, and there is a large problem in the measurement precision and reliability in a high frequency region. At the same time, the CT having a superior high frequency characteristic is generally expensive. At the same time, in the current measurement by the CT, a size of (a casing of) the CT becomes a problem. As a result, these factors obstruct size reduction and cost reduction of the device.
In addition, in a measurement method using the shunt resistor, enormous energy lost as a heat when a large current flows becomes a problem and insulation measures are necessary in many cases. Similar to the case of the CT, these factors obstruct the size reduction and the cost reduction of the device.
Meanwhile, in the related art, an electric current measurement apparatus using a magnetic element is known as an electric current measurement apparatus that does not use the shunt resistor and is relatively cheap. In the present specification and claims, the “magnetic element” is an element that changes an electrical characteristic by a magnetic field from the outside of the element. Therefore, a Hall element using a semiconductor is also handled as the magnetic element.
The magnetic element detects a magnetic field generated by a current. Therefore, current measurement using the magnetic element becomes technology called magnetic field detection widely. A clamp-type ammeter is representatively used. This is inserted into a magnetic circuit in which a conductor to which a current flows is configured using a magnetic material and a magnetic field generated by the current is detected by the magnetic element (Patent Literature 1).
In addition, the invention in which power consumption in a measurement target circuit is measured by the magnetic element is reported (Patent Literature 2). In addition, a magnetic field measurement apparatus using a magnetic impedance effect is reported (Patent Literature 3). The magnetic impedance effect is an effect in which, when an alternating current is flown to the magnetic element and a flow of the current is unevenly distributed on only a surface of the magnetic element by a skin effect, impedance of the magnetic element changes at high sensitivity with respect to an external magnetic field.